Liquid crystal displays in which amorphous silicon is used in a channel layer of a thin film transistor has been mainstream of liquid crystal displays driven by using thin film transistors. However, it is gradually becoming difficult for the channel layers formed with amorphous silicon to achieve high quality required for liquid crystal displays. Therefore, in recent years, oxide semiconductors have received much attention as alternative channel layer material to amorphous silicon.
Oxide semiconductors can be deposited to form films by the sputtering method, unlike amorphous silicon that is deposited to form films by the chemical vapor deposition method (CVD). Therefore, oxide semiconductors have excellent film uniformity, and have the potential that can deal with the desire for increasing the size and resolution of liquid crystal displays.
In addition, since oxide semiconductors have higher carrier mobility than that of amorphous silicon, oxide semiconductors are not only advantageous in terms of high-speed switching of images, but also can be expected to reduce power consumption (achieve power saving) since leakage current is extremely low during OFF state.
Furthermore, since the sputtering method enables formation of a film at a lower temperature than that in the chemical vapor deposition method, the sputtering method is advantageous in that it allows selection of materials having poor heat resistance as materials constituting thin film transistors.
Indium-gallium-zinc composite oxides (hereinafter, referred to as “IGZO”) and zinc-tin composite oxides (hereinafter, referred to as “ZTO”) are known as examples of oxide semiconductors suitable for channel layers of liquid crystal displays.
As techniques related to the foregoing, n-type thin film transistors using IGZO are disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-165532). Also, targets formed of a ZTO sintered body and oxide semiconductor films using the same are disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2010-37161, JP-A No. 2010-248547, and JP-A No. 2012-33699, and Toshio Kamiya and two others, Solid State Physics, AGNE Gijutsu Center Inc., Sep. 15, 2009, vol. 44, No. 9, p. 630-632).
Furthermore, it is known that oxide semiconductors such as IGZO and ZTO have a tendency that their properties as semiconductors deteriorate when exposed to light such as ultraviolet light, and that TFT characteristics thereof deteriorate when TFTs are prepared therefrom (see, for example, Kazushige Takechi and four others, Japanese Journal of Applied Physics, The Japan Society of Applied Physics, Jan. 20, 2009, vol. 48, p. 010203-1-3, and P. Goerrn and three others, Applied Physics Letters, (United States), American Institute of Physics, Nov. 6, 2007, p. 193504-1-3).